1. Field of the Invention
The invention relates to a battery balance circuit, and more particularly, to a battery balance circuit implemented in a battery with a number of cells connected in series and capable of balancing the power between different cells.
2. Description of the Prior Art
Most electronic devices use rechargeable battery as a power source for its advantageous convenience and capacity, in which batteries using lithium polymer as core substance are regarded as the most mature products with high capacity density specification. A rechargeable battery is primarily charged by a power supply unit or via an AC adapter from an electronic system where the rechargeable battery is installed.
The rechargeable battery is usually made of cells, each with specific capacity and connected the each other in series. During the charging or discharging process of the cells, imbalance between cells often takes place due to the state of each cell, which would shorten the life and decrease the usable capacity of the cells. Conventionally, some measures have been taken to balance the cells in an attempt to extend the life and maintain the usable capacity of the cells:
Series balance circuit in parallel with resistance. In a number of cells connected in series with one another, each cell is connected with a resistance in parallel and during the charging process, cell with higher voltage is made to consume its own power through the connected resistance. It is apparently a simple and low-cost, but far less efficient balance solution.
B. Series balance circuit with switching inductance. A number of inductances are disposed in a rechargeable battery, each connected in parallel with one of the cells. During the charging process, cell with higher voltage is forced to store the power in the inductance by turning on a switch coupled therebetween and the inductance goes on to release the power to a next cell. Given the limitation that electrons in the circuit may only be conveyed to a neighboring cell, more cells in a battery the poorer efficiency the balance solution gets.
C. Series balance circuit with switching capacitance. A number of capacitances is disposed in a rechargeable battery, each connected in parallel with its neighboring cells via two-way switches. The cells are made to balance through fast turning on and off of the switches. Sharing the same disadvantage as the previous solution, since electrons in the circuit may only be conveyed to a neighboring cell, if the power of a first cell is to be conveyed to a very last cell, through a number of middle cells, the power should have gone through repetitive storing and releasing in every intermediate capacitance. Such long path for conveying the power substantially effect the efficiency of the balance solution.
These solutions for balancing cells in a rechargeable battery all have efficiency issue while great unnecessary power loss is inevitable.